Influence of hybridization and substitution on the properties of the CH center dot center dot center dot O hydrogen bond

Ab initio calculations are used to compare the hydrogen bonding ability of acetylene and ethylene, and various derivatives, with the analogous propert ies of alkanes. Water is used as the universal proton acceptor and paired w ith HCCH, FCCH, CICCH, and NCH as well as with ethylene, and its mono-, di- , and trifluorosubstituted derivatives. With regard to the hydrocarbons, sp -hybridized acetylene forms the strongest bond, followed by sp(2) and then sp(3). Halogenation of the proton donor strengthens each type of hydrogen b ond, particularly when the substitution takes place on the C involved in th e hydrogen bond. sp(3)-hybridized systems are most sensitive to this substi tution-induced bond strengthening, followed in order by sp(2) and sp. For e ach hybridization type, the length of the hydrogen bond shortens in proport ion to the strengthening, with alkanes exhibiting the greatest sensitivity and alkynes the least. Whereas formation of the hydrogen bond causes the sp (3) CH bond to contract and undergo a blue shift, the opposite trend is see n in the alkynes; the CH bond length is essentially unaffected in the alken es. All types of CH..O hydrogen bonds are weakened more gradually as the tw o subunits are drawn apart than are OH...O bonds. Whereas alkyne CH...O bon ds behave very similarly to OH...O interactions with regard to angular dist ortions, the hydrogen-bond energy of alkenes is less sensitive to such nonl inearity.